1. Field of the Invention
The present invention relates to seismic prospecting. More particularly, it relates to a system for increasing the efficiency of data gathering operations.
2. Description of the Prior Art
In the past, the most common method for generating elastic wave energy for seismic prospecting has been to generate a sharp energy pulse using a source such as dynamite. In this method of seismic prospecting, all of the elastic wave energy is injected into the earth at substantially the same time, and the seismic energy reflections from subsurface reflecting interfaces are detected at one or more surface locations. It is necessary to wait until all the returning seismic signals have been recorded before generating the next energy pulse so that a given seismic recording will not contain reflected signals from two sources, thus making it difficult to determine the reflecting interfaces responsible for the signals.
Hydraulically or electromagnetically driven vibrators are now commonly used for generating seismic signals. Reflected seismic energy from subsurface reflecting interfaces is detected by geophones or other seismic detectors, and travel time of the seismic wave is determined by cross-correlating the detected signal with the transmitted signal. Because the seismic energy level generated by such vibrators is low, a long vibratory signal must be generated to impart sufficient energy for the signals resulting from reflections from subsurface interfaces to be distinguishable from noise. It is well known to those skilled in the art that to determine the depth of a particular reflecting interface, the injected seismic signal must be unique (i.e., nonrepetitive) for a time at least as long as the longest travel time to be measured.
One method for developing sufficient reflected energy is to inject a plurality of vibratory cycles into the earth. Following each cycle, vibrations cease until such time as all the reflected energy of interest has returned to the surface. After the reflected signals from each cycle are recorded, a new cycle is initiated. The signals representing the detected energy from each vibratory cycle are "stacked" or summed with signals from previous cycles. This method is similar to seismic prospecting with dynamite in that a signal is generated and then signal generation stops until all reflections of interest have been recorded and then another signal is generated.
Another method of generating sufficient energy is to vibrate continuously long enough so that the detected signal, when correlated with the transmitted signal, has enough energy for seismic reflections to be distinguishable from noise. This method requires less prospecting time than the method wherein a plurality of shorter vibratory cycles are transmitted, but more recording and storage space is needed to store the transmitted and received signals.